Is it merely coincidence that every time I hear about Phoenix it's from the Rails community? Is it popular with people who haven't come to it from Rails?
The Elixir community may be an amalgamation of Rails folks looking for better runtime performance and cool new tech, and Erlang folks looking for syntactic sugar and a growing developer community. (I'm neither.)
Elixir's author was one of Rails core contributors, which probably also helps in bringing the Rails crowd over. One could think of Elixir as a "better" version of Rails rewritten on top of the Erlang stack, even if Rails and Elixir are apples and oranges under the hood.
See also: the inverse of OP's article, the creator of Erlang trying out Elixir (2013) [1]
[1] http://joearms.github.io/2013/05/31/a-week-with-elixir.html#...
* edited for spelling